776 ECOLOGY 



is the incipient host. If some substances exist in greater concentration 

 in one plant and other substances in the other plant, the condition might 

 give rise to reciprocal parasitism (p. 786). A second stage might be 

 the development of haustoria, followed, perhaps, by a reduction in the 

 number of root hairs and finally by their complete elimination. A third 

 stage might be the development of carbohydrate absorption, apparently 

 corresponding to the first stage of saprophytism; this in turn might be 

 followed, as in thallophytes, by the cessation of food making by the 

 chlorophyll, or even by the entire elimination of chlorophyll and of the 

 leaves as conspicuous organs. Finally, there might come the develop- 

 ment of protein absorption through sieve tube contact, the loss of ger- 

 minating power in the seed except in contact with the host, and the 

 development of a univorous habit. Parasites on aerial organs scarcely 

 can have had a similar origin; the ancestor of the mistletoe may have 

 been an epiphyte and that of the dodder, a vine with adventitious climb- 

 ing roots. In the tropics the mistletoe family has many parasitic repre- 

 sentatives that are scarcely distinguishable from epiphytes, since they 

 possess roots which spread over the bark of the host plants. 



Attempts have been made to ascertain the factors involved in parasitism by 

 growing one plant on another instead of in the soil. Peas have been grown on beans 

 in such a way that they could obtain water and salts only from the latter; under 

 these conditions they not only developed a root system within the artificial host, 

 but reached maturity, producing flowers and even seeds capable of germination. 

 Similar and equally successful experiments have been carried on for a much longer 

 time in which a grape {Cissus digitala) has been used as the parasite and various 

 cacti (as Opuntia and Cereus) as hosts. In this experiment the grape probably 

 is a partial parasite comparable to the mistletoe, taking chiefly water and salts from 

 the host and manufacturing its own carbohydrates. This conception is supported 

 by the fact that the osmotic pressure of the cell sap of the grape was considerably 

 higher than that of the cactus. In all cases studied the osmotic pressure of the 

 sap of the experimental parasite surpassed that of the host. 



Grafting. — General phenomena. — Closely allied to parasitism are 

 the phenomena of grafting, a process in which a shoot or bud (the scion) 

 is inserted in another plant (the stock) in such a way that the former 

 continues to grow through the use of food materials derived from the 

 latter. Grafting is done in various ways, one being to place together 

 oblique surfaces of stock and scion of about equal size, grafting-wax 

 being employed to hold them in place and to give protection; or small 

 scions may be inserted in a large stock which has been cut transversely 

 (figs. 1087, 1088). Budding is a form of grafting in which a single bud 



